1. Field of the Invention
This invention relates generally to the field of ocular surgery, and more particularly to the use of excimer lasers for corneal refractive and therapeutic surgery.
2. Description of the Related Art
The use of excimer lasers in ocular surgery is today well known for performing corneal ablations with a high degree of accuracy. Applications of the excimer laser in refractive surgery include corrections of myopia, hyperopia, astigmatism, and presbyopia through the ablation of tissue on or within a patient""s cornea. Thus, in photorefractive keratectomy (PRK), the aim is to flatten or steepen the central cornea to eliminate myopic, hyperopic, or presbyopic refractive errors and to correct corneal astigmatism. In phototherapeutic keratectomy, on the other hand, the intent is to smooth irregular corneal surfaces or remove opaque corneal tissue. A normal cornea is shown on the anterior portion of the eye, also known as the ocular globe, in FIG. 1.
The cornea includes five distinct layers, as seen in FIG. 2. Outermost layer 1 is the Epithelium, which is an anterior surface layer that promotes constant and active cellular reproduction. Second layer 2, known as the Bowman has a structural function of serving as an attachment means for the corneal epithelium. The Bowman layer adheres anteriorly to the epithelium and posteriorly binds to third layer 3, which is called the stroma. The stroma is the thickest layer, which gives body to the cornea. Fourth layer 4, called the Descemet, posteriorly bounds the stroma. The innermost and fifth layer 5 is known as the Endothelium, and includes a layer of immunological cells whose function is to pump-dehydrate the cornea and keep it transparent.
The excimer laser used for ocular surgery is typically a 193 nanometer Argon Fluoride surgical laser system, such as the Schwind Keratom System. FIG. 3 illustrates a schematic of laser beam delivery system 14 for the Schwind Keratom System. The Argon Fluoride excimer laser beam first passes through several optical components 18 where it is collimated and aligned to the eye to be treated. These optical components, which must be transparent for conducting the ultraviolet radiation of the beam, are made from a synthetic quartz such as Suprasil II.
Mechanical shutter system 16 controls the emission of the excimer laser beam. The shutter system consists of two independently actuated shutter blades that block the laser beam if the shutters are not energized. When the shutters are energized to permit passage of the laser beam, which is originally rectangular and exhibits a cross-section of approximately 8xc3x9724 mm, the beam is directed by 90-degree bending mirror 20 onto a specially designed optical component called an integrator, referenced as 22. The integrator homogenizes the laser beam energy over its cross-section.
The uniform and homogenous rectangular laser beam is then passed through a series of beam-stops 24 on moving steel band 26 to create the intended diameter of the laser beam. After passing the beam-stops, the laser beam is bent downwardly by dicroitic beam-splitter 28 for presenting the beam to the fixation target, the patient""s cornea 10. The beam-splitter also permits the patient""s eye to be observed via video camera 30, as indicated in FIG. 3.
After exiting beam splitter 28, the laser beam passes through another lens system 32 before hitting the patient""s cornea. The lens system consists of two large-diameter lenses that produce an image of the beam-stop onto the patient""s cornea. These lenses thus act like a zoom projection system.
The operation of integrator 22, steel band 26 with beam-stops 24, and zoom projection system 32 is computer controlled for precise variation of the corneal ablation diameter, typically over a range as wide as 0.6 to 8.0 mm. The corneal ablation results from the energy and wavelength of the laser beam, which disrupts the bond between molecules in the cornea and destroys corneal tissue in a controlled manner.
The system does not work in the same way for phototherapeutic keratectomy and photorefractive keratectomy. For phototherapeutic keratectomy, the surgeon selects a certain diameter of the ablation zone. This means that only one aperture of the steel band is used, and that the two large-diameter lenses do not move during the ablation process.
In the photorefractive keratectomy mode, however, the computer-controlled system changes the laser fluence during the treatment, depending on the selected steel band aperture and the position of the zoom optic. The zoom range and the available diameters of the beam-stops are designed so that a continuous variation of the ablation diameter of the cornea should be possible. For example, a simple myopic ablation of the patient""s cornea may be achieved by a computer controlled combination of changes of different beam-stops and movements of the two large-diameter lenses.
Surgical lasers, such as the Schwind Keratom system, are controlled with the aid of nomograms that are based upon numerous studies and data bases. Such nomograms allow for appropriate correction of refractive defects in most patients, once the patient""s particular gradation of myopia, hyperopia, astigmatism, or presbyopia has been identified through keratometric and subjective examination.
Refractive corneal surgery with an excimer laser is presently conducted in one of two ways: photorefractive keratectomy (PRK), shown in FIG. 4, and Laser In situ Keratomileusis (LASIK), shown in FIG. 5.
In the PRK technique, laser beam 40 is applied directly to the patient""s corneal surface 10 according to the particular refractive defect. The laser system thus destroys an anterior portion of the cornea according to the machine""s nomogram, leaving the stroma uncovered and resulting in the change shown at 42 in FIG. 4A. The stroma will later be covered with new Epithelial cells during the healing process, which takes a few days. One of the shortcomings of the PRK technique is that the Bowman layer or membrane is destroyed by the direct corneal application of the laser beam. The destruction of the Bowman layer is a major concern due to that layer""s role in maintaining corneal transparency while serving as point of adhesion for the epithelium.
The LASIK technique involves the use of a microkeratome (not shown), which makes an access cut across the anterior portion of the cornea. More specifically, the microkeratome makes a lamellar resection of the cornea to create a corneal flap and hinge, as seen in FIG. 5. Flap 44, also known as a xe2x80x9cpediculado,xe2x80x9d includes corneal tissue from the Epithelium, Bowman, and anterior stromal layers. The corneal flap is typically circular, having a diameter between 7 and 9 mm, and averages about 160 microns in thickness. A presently preferred microkeratome is described in pending U.S. application Ser. No. 09/002,515, the entire contents of which are incorporated herein by reference.
The corneal flap allows corneal stroma 3 to be exposed for ablation by laser beam 40 that is appropriate to correct the patient""s refractive defect, as indicated in FIG. 5A. This results in altered stromal region 46 shown in FIG. 5B. Following the laser application, corneal flap 44 is returned to its initial position, as shown in FIG. 5C, and, since no ablation has been performed at the corneal surface, the patient suffers no destruction of the Bowman layer and recovers very rapidly.
A technique similar to the present-day LASIK procedure was described as early as Jun. 20, 1989 in U.S. Pat. No. 4,840,175 to Gholam A. Peyman. The ""175 patent teaches the use of an excimer laser to modify the curvature of a patient""s cornea. A thin layer of the cornea is removed xe2x80x9cby cuttingxe2x80x9d (no details are provided), and a laser beam is then applied to either the thin layer or the exposed corneal surface. A variable diaphragm is used to form the laser beam into a desired predetermined pattern for ablation of desired portions of the thin layer or exposed cornea. The ""475 patent further discloses the formation of disc-shaped and annular-shaped ablations for decreasing and increasing the curvature of the cornea as is desired.
The LASIK technique is particularly described for correction of presbyopia in U.S. Pat. No. 5,533,997. The ""997 patent describes the use of a xe2x80x9ccorneal shaper,xe2x80x9d a particular microkeratome, in combination with an excimer laser to first resect at least a portion of the patient""s cornea and then ablate an annular portion of the exposed stroma. The annular ablation is formed either by directing the laser beam in a circular path or by placing a mask over the central area of the stroma.
It is also generally known that in the art of refractive surgery various types of corneal topographers are used for mapping various portions of the cornea. Thus, the use of corneal topographers can provide information on the surface configuration of the corneal tissue by taking advantage of the cornea""s transparency and analyzing light reflection and refraction on the cornea. The information is provided as a color scale representation of the corneal surface(s) with different values for interpretation. Once interpreted, the topographical information is useful for detecting which corneal pathologies can be corrected and which are not correctable by present refractive surgery techniques, and also for centering the laser beam ablation on the patient""s visual axis.
Corneal topographers also permit the study of the cornea after refractive surgery has been performed. The normal cornea is somewhat aspherical with one meridian being flatter than the other. The resulting topographical map is shaped like hourglass 50, as shown in FIG. 6. This may be contrasted with central circular pattern 52 of FIG. 6A resulting after surgery to correct for myopia, resulting oval pattern 54 of FIG. 6B after surgery to correct for astigmatism, and resulting periferical pattern 56 of FIG. 6C after surgery to correct for hyperopia.
FIG. 7 illustrates an example of a particular corneal condition, called Corneal Ectasia and referenced at 58, in which the Stroma contains anomalous collagen which prevents the cornea from maintaining the normal aspherical shape and causes asymmetrical refractive defects that are very difficult to correct with glasses or contact lenses. Such Corneal Ectasia is virtually impossible to correct with either PRK or LASIK because these techniques further weaken the already distressed corneal structure.
Other corneal defects, including Leucoma scars (FIG. 8) and/or surface irregularities (FIG. 9) have not been successfully treated with conventional methods. Such defects have, for example, been treated by conventional manual resection, using a spatula or knife, of the anterior portions of the afflicted cornea and grafting tissue from a donor cornea onto the ablated area. These techniques, called Lamellar Keratoplasty or Homoplastic Lamellar Keratomileusis, spare the descemet or endothelium layers, since these layers are not affected by such pathologies. However, Lamellar Keratoplasty has lately been abandoned for use in these pathologies, because opacities and/or irregularities are typically left on the contact surfaces between the donor graft and the receiver stromal bed which prevent the patient from obtaining adequate vision recovery.
Patients suffering from corneal ectasia, leucoma scars, thin areas or corneal irregularities may otherwise find adequate vision correction through penetrating or deep keratoplasty, which is a total corneal depth transplant. Thus, the entire cornea is removed, including all five layers, and is replaced with an entire donor cornea as indicated in FIG. 10.
Several dangers and disadvantages are at risk when performing a deep keratoplasty, including the following:
1. the donor cornea must be fresh and contain a high number of active endothelial cells, since these cells are responsible for pump-dehydrating the cornea and also ensuring the final transparency of the graft tissue;
2. the complete healing of the donor graft requires approximately six months, during which time the sutures should not be removed;
3. the graduation of astigmatism is generally high, and can increase depending on the tension with which the sutures are left in place;
4. there is a relatively high likelihood of the donor graft failing and experiencing opacification due to endothelial rejection; and
5. it is an invasive technique involving perforation of the ocular globe, and thus presents danger of intraocular structural damage, loss of vitreo, intraocular infection, and in the worst case expulsion of the eye contents.
To address these disadvantages and shortcomings in the art, it is a principal object of the present invention to provide a surgical method that is based upon information about the shape of the corneal structure rather than merely the refractive defect identified by keratometric and subjective examination.
It is a further object of the present invention to combine corneal topography systems, particularly elevation or altitude topographers, with excimer laser systems to perform corneal ablation based on information regarding the corneal anterior and posterior surfaces.
It is a further object of the present invention to achieve the ablation of greater amounts of tissue in some corneal areas and smaller amounts of tissue in other corneal areas, as indicated by elevation topography.
It is a further object of the present invention to ablate a portion of the cornea so as to leave the posterior layers of the cornea with an even thickness.
It is a further object of the present invention to achieve corneal ablation while protecting the descemet and endothelium layers.
It is a further object of the present invention to allow the performance of a Lamellar Keratoplasty that achieves a known, smooth, accurately estimated stromal bed that prevents irregularities in the final outcome such as opacities of the contact surfaces between the donor and receptor surfaces.
The objects described above, as well as various other objects and advantages, are achieved by a method and system for controlling the depth of corneal ablation during optical surgery. The method and system contemplate the steps of mapping the anterior and posterior surfaces of a patient""s cornea to obtain full pachymetry of the patient""s cornea. The patient""s corneal pachymetry is used to identify a desirable corneal bed, and to guide a surgical laser in ablating a portion of the anterior surface of the patient""s cornea to produce the desired corneal bed.
The corneal pachymetry may be used to identify whether the cornea exhibits refractive defects, therapeutic defects, or both.
The depth control method and system of the present invention contemplates the desirable corneal bed being substantially beneath a defective portion of the cornea, whereby ablation of the defective portion of the cornea would expose the desirable corneal bed.
The surgical laser is guided so that the resulting ablated portion of the patient""s cornea may exhibit either a constant thickness or a variable thickness as determined by the patient""s corneal pachymetry. In this fashion, the thickness may be varied for correction of myopia, hyperopia, astigmatism, ectasia, leucoma, or corneal surface irregularities.
The above-described method and system may be useful in various applications, such as lamellar keratoplasty or deep keratoplasty. In such applications, a corneal disc is ablated from the anterior surface of a recipient cornea in such a manner that the donor corneal disc has a similar diameter and thickness to the ablated portion of the patient""s cornea, or a known, greater, or lesser diameter, as the surgeon may determine for that specific patient. The donor corneal disc is then placed onto the produced desired corneal bed of the patient""s cornea.
It is preferred, in applications such as lamellar keratoplasty or deep keratoplasty, that the thickness of the ablated portion of the patient""s cornea is designed such that the descemet and endothelium layers of the cornea are left intact.
It is also preferred in the above-described method and system that the mapping step be accomplished with an elevation-type corneal topography system, such as the ORBSCAN topography system.
The present invention further contemplates a method and system of ocular surgery that improves on the LASIK procedure given that it can be performed on patients that would not otherwise be candidates for LASIK. First, the anterior and posterior surfaces of a patient""s cornea are mapped to obtain full pachymetry of the patient""s cornea. A desirable corneal bed is identified using the patient""s corneal pachymetry. The patient""s ocular globe is secured, and on the basis of the topographical information, an ablation is made with the laser of the amount of tissue to be removed in order to obtain a known adequate corneal bed. The procedure is completed by replacing a lamellar or total known thickness and known diameter donor cornea on the bed so prepared.